### Universe Song, quantitatively evaluated

Something reminded me recently of the Universe Song from the 1983 Monty Python movie, The Meaning of Life. You’ve seen that clip, right?

It’s a funny song and a funny sketch. It also is, unlike certain science fiction TV shows, reasonably accurate with its facts and numbers. You quite literally could use nothing but your knowledge of this song to stand a good chance of passing an Astronomy 101 final exam. Shall we fact check?

… a planet that’s evolving
And revolving at nine hundred miles an hour,

Not only is Earth evolving — it’s still cooling, for one thing, and the continents are drifting around, and the magnetic field flips every once in a while — but it is rotating (“revolving” refers to its motion around the Sun, but I quibble), and while one usually uses angular velocity measures such as radians per second or hours per revolution or something, one certainly can observe that the Earth’s circumference at the Equator is 24,873.6 miles (source) and that relative to the fixed stars, Earth rotates once in 23.934 hours (same source). So the speed at which Earth’s surface moves at the equator is 1039 miles per hour. The song’s value is low by 13%. Not too shabby, though “a thousand miles an hour” would have scanned just as well and been more accurate.

That’s orbiting at nineteen miles a second, so it’s reckoned,

Likewise, Earth’s orbital circumference and sidereal revolution period are 584,019,311 miles and 365.26 days, for an orbital speed of 18.51 miles per second. Song is high by a mere 2.7%.

A sun that is the source of all our power.

Not quite true but close. Solar power comes from the Sun, of course; wind power comes from air circulation driven by the Sun (and Earth’s rotation); water power comes from water flowing downhill after solar evaporation lifted it up; fossil fuels come from ancient life that either photosynthesized using sunlight or fed on life that did. Nuclear and geothermal energy, though, does not have a solar origin. Not our Sun, anyway.

The sun and you and me and all the stars that we can see
Are moving at a million miles a day

The Solar System is about 28,000 light years from the center of the Milky Way, and it revolves around that center once in about 230 million years (source).  Doing the math, that comes out to 12.3 million miles per day.

But the song apparently isn’t referring to that speed, but the slower speed with which the Solar System is moving toward Lambda Herculis, in a reference frame in which the other stars are, on average, not moving: about 12 miles per second (source). In other units, that’s 1.0 million miles per day — exactly (to within stated accuracies) what the song says.

But the previous line’s wrong: that’s the Sun’s speed relative to the other stars, not the speed of “all the stars that we can see” relative to… something. Well, they’re moving a million miles a day relative to us, but that’s stretching the interpretation.

In an outer spiral arm, at forty thousand miles an hour,

I’d prefer to say “the outer part of a spiral arm”, but yes. And that’s just the same speed again in different units; 12 miles per second is 43,200 miles per hour. The song’s value is 7% lower.

Our galaxy itself contains a hundred billion stars.

That’s the right order of magnitude (source). I’ve seen 200 to 400 billion claimed (source). Large uncertainties here due to the number of dwarf stars which are hard to detect.

It’s a hundred thousand light years side to side.

It bulges in the middle, sixteen thousand light years thick,
But out by us, it’s just three thousand light years wide.

And also the right order of magnitude, with this source giving about 10,000 light years for the thickness of the central bulge and 1000 light years for the main disk. There’s considerable uncertainty in these numbers — the Milky Way is harder to view than many other galaxies, because we’re in it and dust obscures a lot of it.

We’re thirty thousand light years from galactic central point.
We go ’round every two hundred million years,

We’ve covered those numbers already, quoting 28,000 light years and 230 million years. So they’re correct to within about 15%.

And our galaxy is only one of millions of billions

Another number that’s hard to estimate, but one source quotes 100 to 200 billion galaxies in the observable universe. “Millions of billions” is a whopper of an overestimate… except that the observable universe is likely only a small fraction of everything there is, the size of which we have no way to estimate with any precision, and it may well be infinite, in which case this is a whopper of an underestimate.

The universe itself keeps on expanding and expanding

Well, yes.

As fast as it can go, at the speed of light, you know,

Well, no. Relative expansion velocities depend on distance and can be less or more than the speed of light. But the bounds of the observable Universe are governed by the speed of light: it’s the region in which light has had enough time to reach our eyes.

Twelve million miles a minute, and that’s the fastest speed there is.

It’s the fastest speed at which a particle or signal can propagate through space (according to present understanding); space itself, as I alluded to above, can expand faster. The speed of light in vacuum is defined to be exactly 299,792,458 m/s (source) (or in other words, the meter is defined to be 1/299,792,458 the distance light travels in a vacuum in a second). In other units, that’s 11,176,943.8 miles per minute. The song’s value is 7.4% too high. But “eleven million” wouldn’t have scanned.

And pray that there’s intelligent life somewhere up in space,
‘Cause there’s bugger all down here on Earth.

This is correct (source).